1. Field of the Invention
The present invention relates to the manufacturing of plastic articles, and preferably to those formed by extrusion, and in particular to an energy and raw material savinqs method, which renders a plastic product having a more even quality than is available today.
The invention is primarily intended to be used in conjunction with the forming of plastic articles by extrusion, such as blown film and foil, and in such a case, also in combination with other articles, such as plastic coated paper and cables. Persons skilled in the art of plastic extrusion do however realize that the invention is useful for many various types of extrusion processes. In the following, however, it will be described in conjunction with manufacturing of a plastic foil using film blowing technique, without limiting the inventive scope.
The invention permits a higher productive capacity with a lower specific energy consumption than is possible to achieve today, at the same time resulting in a final product with excellent dimension stability and constant mechanical and optical characteristics.
2. Description of the Prior Art
The conventional technique used today for manufacturing plastic articles, utilizes an extrusion machine or extruder. This machine is supplied with plastics in the form of granules or in another disintegrated form, possibly along with additives, such as UV-stabilizers, colour agents and similar. The feed stock is processed in the machine by means of a rotative screw, which also operates the outfeed of the plastic, to a viscous condition. The energy provided by the screw rotation melts the feed stock, but in so far as there is an additional wish to make the plastic more mobile, i.e. lower its viscosity, additional heat from special heating elements may in certain cases be supplied. From the extruder, the viscous plastic melt is fed, directly or via additional machines, to nozzles or dies for forming the actual plastic article.
The flights of the screw do not operate with an absolute seal against the surrounding cylinder wall, but a certain material return flow occurs between them. This improves the homogenisation and the raw material mixture of the molten material before it is fed into the nozzle or die by intermediary means of a screen pack assembly, which strains contaminants in the melt.
As long as the extruder screw is in a good condition, the machine usually operates with a relatively even throughput. There is, however, a wear and tear on the screw flights making the angular gap between them and the surrounding cylinder wall progressively bigger. The pressure drop in the screen pack assembly increases with the fineness of the sieve cloth and with the degree of its clogging, because of the impurities in the melt sticking to the sieve cloth. To be able to maintain an even melt throughput from the screen pack assembly, the pressure of the melt has to be increased before said screen pack assembly. This can only occur by increasing the number of screw revolutions, which in turn increases the return flow through said angular gap. The net outcome will then be the following:
A. The specific throughput through the extruder decreases.
B. The added specific mechanical energy results in an increased melt temperature before the screen pack assembly, with a resulting viscosity decrease of the melt, which further decreases the specific throughput due to increased return flow, and a further increase of the specific mechanical energy supply etc. until an equilibrium is reached.
C. The increased return flow implies in addition that the molecule chains of the polymers are subjected to an increased specific portion of shearing force resulting in a desintegration and shearing-off of the molecule chains. This leads to a drastic change of the polymer mechanical and optical characteristics after crystallization. A shearing-off of the molecule chains will also lower the viscosity further, which increases said return flow.
D. The increase of the melt temperature implies an increased cooling capacity, so that a constant frostline height is obtained, i.e. the line where crystallization occurs after or above the die. The position of the frostline is of importance from a quality view point with respect to both optical and mechanical properties of the formed plastic article. If the frostline height above the nozzle or die increases, which means insufficient cooling, the production rate has to be decreased. In other words, the frostline position is of importance for the production per time unit.
Keeping the mechanical energy supply per melt unit constant, the common technical term being "kinetics of extrusion", is therefore of a considerable significance for a final product with reproducible and constant mechanical and optical characteristics to be manufactured.
From what has been described above, it is evident that the level of kinetics of extrusion will be directly affected, if the production rate of a given production line is changed. This implies variations of utilized raw materials, as well as variations of machine equipment and its drive units, all of which effects the quality characteristics of the final product by varying the optimal level of kinetics of extrusion. Up to the present, an accurate control of the level of kinetics of extrusion has not been possible, and the processes have been run more on empirical experience and a desire for as high a production rate as possible. The level of kinetics of extrusion has varied much and has usually been too high to result in a high production rate with an acceptable production cost level.
With the increasing need to obtain higher production from each production unit, attempts have been made to lessen the need for a great pressure build-up after the extruder, either by designing dies with a lower pressure requirement or by supplementing the extruder with a special feed pump and in certain cases several of such pumps, for example one or several gear pumps. This pump provides the pressure build-up and an even and pulsation free outfeed to the die. This will in turn permit a greater production per time unit than the case is when such a pump is absent, and thus a higher production. The pump, however, does not solve the principle problem, which is to maintain a desired and constant level of kinetics of extrusion, independent of variations in the raw material, in the flowthrough of the screen pack assembly and/or in the production per time unit.